"I think that we will have two rounds and that Paluckas and Sinkevicius will get through to the runoff," Butkevicius, who is not seeking re-election, told reporters on Thursday, the first day of voting in the party's first-ever direct leadership election.
Five candidates are running for the post.
Ramūnas Karbauskis, the leader of the Lithuanian Farmers and Greens Union (LFGU), the LSDP' larger coalition partner, told reporters that the results of the Social Democratic leadership election would have no impact on the future of the coalition.
After the Social Democrats finished third in the elections with only 17 seats in the 141-membner Seimas, Butkevičius in early November announced his intention to resign, but the party's council did not approve of the move.
The Social Democrats are electing their leader in a direct election for the first time. Its chairpersons were until now elected by the party's congresses.
The LSDP is Lithuania's largest political party with over 20,000 members.
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